This invention relates to a laminated plate electron multiplier comprising a stack of conducting sheet dynodes insulated from one another, channels passing transversely through the stack from an input dynode to an output dynode, each channel comprising aligned apertures in the dynodes, the maximum cross-sectional dimension of all the apertures being substantially the same, and at least the walls of the apertures having an exposed secondary electron emissive surface, and means for enabling a repelling field to be provided in the vicinity of the outer surface of the input dynode.
The invention also relates to a cathode ray tube including a laminated plate electron multiplier.
Such channel plate electron multipliers and methods for manufacturing them are described in British Patent Specification No. 1,434,053. In use, the dynodes are held at progressively increasing positive d.c. voltages from input to output. Electrons falling upon the wall of the hole of the input dynode of a channel give rise to an increased number of secondary electrons which pass down the channel to fall upon the wall of the hole of the next more positive dynode where further secondary emission multiplication occurs. This process is repeated down the length of each channel to give a greatly enhanced output electron current substantially proportional to the input current.
Channel plates may be used for intensification of electron images supplied either by the scanning, for example raster scanning, of the electron beam of a cathode ray tube or by a photocathode receiving a radiant image which excites photoelectrons which are fed as a corresponding electron image to the input face of the channel plate. In either event, electrons fall on the portions of the input face of the first dynode of the channel plate between the channels, exciting secondary electrons which, by reason of their spread of emission energy and direction, pursue trajectories in the space in front of the channel plate which can carry them into channels remote from their point of origin. The contrast and definition of the image are degraded by each channel receiving additional input electrons in proportion to their original input electron density at channels over a range of distances away.
The sheet dynodes may be made from a metal alloy such as aluminium magnesium or copper beryllium which is subsequently activated by heating in an oxygen atmosphere to produce a surface all over the dynode which has a high secondary emission coefficient. The input face will thus have an undesirably high secondary emission leading to contrast degradation. Alternatively, the dynodes may be made from sheet steel coated with cryolite, for example, to give a secondary emission coefficient of 4 or 5. In this case also it is impractical to restrict the coating of cryolite to the insides of the holes, and the input face will again have an undesirably high secondary emission coefficient.
British Patent Specification No. 2,090,049A discloses using a mesh-like grid to produce a positive or negative electric field in front of the input face of the input dynode for the purpose of reducing the effect of input electrons striking the secondary emitting surface between the apertures and producing unwanted secondary electrons which spread across the surface of the input dynode and may enter channels remote from their point of origin thus spoiling the contrast and definition of an image to be displayed. Although the electric field produced by the mesh-like grid has been shown to be effective in contributing to the improvement in contrast and definition, there is still a desire to seek a further improvement.
In this connection British Patent Specification No. 2,080,016A discloses improving the contrast of a laminated channel plate electron multiplier by providing a layer of material having a secondary electron emission coefficient less than 2.0 on the outermost surface of the input dynode between the convergent apertures in the input dynode. Conveniently, the material is carbon and is deposited on an apertured carrier sheet placed in contact with said outermost surface. This layer reduces the number of unwanted secondary electrons which are produced but it does not eliminate them. The production of the positive or negative electric fields as disclosed in Patent Specification No. 2,090,049A can be used to advantage with an input dynode having a layer of a material having a secondary emission coefficient less than 2, on the outermost surface between the apertures to reduce the spread of unwanted secondary electrons.
While these techniques go a long way to reducing loss of contrast due to the production of large numbers of secondary electrons from the surface between the apertures at the outermost side of the input dynode, they are less effective in preventing stray secondary electrons from escaping from the inwardly convergent periphery of each aperture in the input dynode and either entering an adjacent channel or not entering a channel at all. The failure of secondary electrons to enter their associated aperture means that the gain of the channel is diminished and that in the case of spatial information it is not displayed accurately.